The present invention relates to polyimide precursor compositions and their uses, particularly for producing insulating coatings, dielectric films, smoothing coatings, protective coatings and for numerous applications in the electrical and electronic industries. It more particularly relates to compositions formed by preparing a mixture of autocondensable oligomers and monomers, precursors of thermostable polyimides, with at least one compound having a hydroxyl function chosen within the group of hydroxylated compounds defined hereinafter.
These compositions or solutions of these compositions in a solvent or a mixture of solvents have the advantage of being much more stable than the conventional polyimide precursor solutions, which are generally polyamid-acids. The latter are very sensitive to any temperature rise, so that it is preferable for them to be kept at below 5.degree. C., as well as to the presence of humidity, which deteriorates the polymer as a result of the hydrolysis of the amid-acid groups. However, the compositions according to the invention are remarkably stable and they can be stored for several months at ambient temperature without any significant changes to the characteristics of these polyimide precursor solutions compared with their initial characteristics.
The invention also relates to the use of these compositions and in particular their use for producing insulating coatings, especially for protecting metals such as iron, steel, aluminium, copper, brass, nickel plated copper, metal alloys, titanium or other substrates, e.g. semiconductor substrates such as silicon, germanium and gallium arsenide. It also relates to the use of these compositions for the production of smoothing layers, especially in the production of integrated circuits or multi-layer interconnection systems.
The compositions according to the invention are particularly suitable for the latter applications making use of dielectric layers or films, whose function is to separate several metallic interconnection levels. For example, the upper structure of an integrated semiconductor circuit can have two interconnection levels. A first layer of conductor elements, e.g. polysilicon, aluminium or tungsten is deposited on the surface of the semiconductor. A dielectric insulating film is then deposited on this first interconnection level. Contact holes are made in the dielectric layer at clearly defined points of the circuit. The second interconnection level is produced by forming a metallic film, e.g. by high temperature evaporation and by etching in said film conductor lines using conventional etching methods. A final insulating film can be added in order to protect the integrated circuit against any contamination by external agents.
The surface of an integrated circuit is not planar, because the active elements are produced by a number of successive etching and deposition operations. It therefore has a plurality of relief or step elements and cavity or trench elements. It is particularly important that the dielectric film deposited on said irregular surface can cover the steps and fill the trenches giving a surface which is as planar as possible. The degree of smoothing as defined by L. B. Rothman (Journal of Electrochemical Society 1980, 127, pp. 2116-2120) is calculated with the aid of the following formula EQU .epsilon.=1-h.sub.2 /h.sub.1
in which .epsilon. represents the degree of smoothing, h.sub.1 the initial height of an e.g. metallic step and h.sub.2 the level difference obtained above said same step following the deposition of the insulating layer. This formula shows that if the final surface is perfectly planar, i.e. h.sub.2 =0, we obtain .epsilon.=1. However, if the final step height is equal to the height of the initial step (h.sub.1 =h.sub.2), there has been no smoothing and .epsilon. is equal to 0. Therefore the degree of smoothing .epsilon. varies between 0 and 1 and the closer the smoothing coefficient to 1 the more planar the final surface.
One of the methods used for producing insulating layers consists of using polyimide resins or polyimide precursors such as acid polyamides. Examples of such products are the product marketed by Hitachi Chemical Co. under reference PIQ 13 or the series of products marketed by Du Pont de Nemours under the name Pyralins. These polymers have excellent film forming characteristics, good dielectric properties and an excellent thermal stability. However, their smoothing power, which is generally between 0.15 and 0.4 is relatively poor. One of the reasons is that the polymer concentration in these solutions is usually below 20% by weight, because beyond this value the dynamic viscosity of the solutions is too great for them to be usable for depositing thin films, e.g. having a thickness below 2 micrometers (.mu.m).
In most applications, the dielectric films are deposited on semiconductors by rotation using a spinner. Under these conditions, it has been shown (S. A. Jenekhe, "Polymers for High Technology", ACS Symposium Series, Vol. 346, 1987, p. 261) that the smoothing powder of a polymer solution is governed by numerous parameters such as the deposited solution volume, the solvent volatility, the polymer concentration, the temperature, the solution viscosity, the rotation speed of the spinner, the rotation time and the behaviour of the polymer during the final treatment of drying and baking. One of these parameters, the dry matter concentration, plays a particularly important part in the smoothing power. This phenomena has been revealed with polyimides or polymethyl methacrylate (D. B. La Vergne and D. C. Hofer, SPIE 1985 Vol. 539"Advances in Resist Technology and Processing II", p. 115). This observation was confirmed by L. E. Stillwagon (Solid State Technology, June 1987, p. 67) by comparing conventional resins with a liquid cycloaliphatic epoxide used without a solvent. The latter compound makes it possible to obtain a very high smoothing level of approximately 0.7 to 0.9. However, phenolic or epoxy resins are only usable as production aids, because they do not have an adequate thermal stability to withstand high temperature metallization operations.
At present, only thermostable heterocyclic polymers and in particular polyimides and polyphenyl quinoxalines can be used as intermetallic dielectrics. It has already been stated that the upper limit of the dry matter concentration in conventional acid polyamide solutions, the precursors of polyimides, was approximately 20%. In order to improve the smoothing characteristics of polyimides, it is indispensable to increase the polymer concentration in order to achieve a maximum value.
Solutions with a higher polyimide precursor polymer concentration have been prepared either by using a delay reagent (U.S. Pat. No. 4,720,539) or by preparing oligomers with a low molecular weight replacing, in the synthesis of the polyimides, the tetracarboxylic aromatic acid dianhydrides by bis(ortho-acid esters), which are much less reactive (European Patent 251,828). These methods make it possible to obtain dry matter concentrations of approximately 30%, but the quality of the films prepared according to the latter method is below that of films obtained from conventional high molecular weight polyamid-acids. One of the reasons is that most of the polycondensation reaction must be performed in the solid phase, following the evaporation of the solvent and that the mobility of the molecules is no longer adequate for ensuring that the antagonistic reactive centers have a high probability of meeting.